Transformerless multi-level medium-voltage uninterruptable power supply (UPS) system

Abstract

Systems and methods for supplying power at a medium voltage from an uninterruptible power supply (UPS) to a load without using a transformer are disclosed. The UPS includes an energy storage device, a single stage DC-DC converter or a two-stage DC-DC converter, and a multi-level inverter, each of which are electrically coupled to a common negative bus. The DC-DC converter may include two stages in a unidirectional or bidirectional configuration. One stage of the DC-DC converter uses a flying capacitor topology. The voltages across the capacitors of the flying capacitor topology are balanced and switching losses are minimized by fixed duty cycle operation. The DC-DC converter generates a high DC voltage from a low or high voltage energy storage device such as batteries and/or ultra-capacitors. The multi-level, neutral point, diode-clamped inverter converts the high DC voltage into a medium AC voltage using a space vector pulse width modulation (SVPWM) technique. The UPS may also include a small filter to remove harmonics in the AC voltage output from the multi-level inverter.

What is claimed is: 
     
       1. A transformerless uninterruptible power supply for an electrical load, comprising:
 an energy storage device; 
 a two-stage DC-DC converter including a first stage that generates a first output DC voltage and a second stage that generates a second output DC voltage higher than the first output DC voltage; 
 a DC-DC converter controller configured to control the first stage with pulse width modulation control signals and configured to control the second stage in a flying mode configuration with fixed duty cycle control signals; 
 a multi-level inverter configured to output a medium AC voltage; and 
 a multi-level inverter controller configured to control the multi-level inverter using space vector PWM control signals so as to perform neutral point voltage balancing, 
 wherein a negative terminal of the energy storage device, a negative terminal of the two-stage DC-DC converter, and a negative terminal of the multi-level inverter are electrically coupled to a common negative bus. 
 
     
     
       2. The transformerless uninterruptible power supply according to  claim 1 , wherein the medium AC voltage is between about 3.3 kV and about 35 kV. 
     
     
       3. The transformerless uninterruptible power supply according to  claim 1 , wherein a positive terminal of the second stage of the DC-DC converter and a positive terminal of the multi-level inverter are electrically coupled to a common positive bus. 
     
     
       4. The transformerless uninterruptible power supply according to  claim 1 , wherein the first stage includes two levels and the second stage includes more than two levels. 
     
     
       5. The transformerless uninterruptible power supply according to  claim 4 , wherein the second stage includes three levels or five levels. 
     
     
       6. The transformerless uninterruptible power supply according to  claim 4 , wherein the two-stage DC-DC converter includes a plurality of switches that form the levels of the first and second stages and a plurality of capacitors coupled together in a flying capacitor topology having a common negative bus. 
     
     
       7. The transformerless uninterruptible power supply according to  claim 4 , wherein the medium AC output is a three-phase AC output,
 wherein the multi-level inverter includes three sets of switches, each of which corresponds to one of the three phases of the three-phase AC output, and 
 wherein each set of switches is configured in a diode-clamped multi-level topology. 
 
     
     
       8. The transformerless uninterruptible power supply according to  claim 1 , wherein the multi-level inverter converts the second output DC voltage into a third output voltage that is an AC voltage smaller than the second output DC voltage. 
     
     
       9. The transformerless uninterruptible power supply according to  claim 8 , further comprising a filter electrically coupled to the AC output of the multi-level inverter and configured to remove harmonics from the AC output of the multi-level inverter. 
     
     
       10. The transformerless uninterruptible power supply according to  claim 9 , wherein the filter is an inductor-capacitor-inductor filter. 
     
     
       11. The transformerless uninterruptible power supply according to  claim 1 , wherein the two-stage DC-DC converter is a bidirectional converter configured to allow a flow of power in a first direction from the energy storage device to the AC output of the multi-level inverter and in a second direction from the AC output of the multi-level inverter to the energy storage device. 
     
     
       12. The transformerless uninterruptible power supply according to  claim 1 , wherein the two-stage DC-DC converter is a unidirectional converter. 
     
     
       13. The transformerless uninterruptible power supply according to  claim 1 , wherein the energy storage device is a low voltage energy storage device. 
     
     
       14. The transformerless uninterruptible power supply according to  claim 13 , wherein the low voltage is between about 700 V and about 1200 V. 
     
     
       15. The transformerless uninterruptible power supply according to  claim 13 , wherein the energy storage device is a battery, an ultra-capacitor, or a battery and an ultra-capacitor electrically coupled to one another. 
     
     
       16. The transformerless uninterruptible power supply according to  claim 1 , wherein the multi-level inverter includes more than two levels.